1. Field of the Invention
The present invention relates to a wavelength division multi/demultiplexer for multi/demultiplexing light, an optical spectrum analyzer for measuring a spectrum of output light from a light source, and an optical bandpass filter for transmitting only a particular frequency component of light, and in particular, to a wavelength division multi/demultiplexer comprising at least two wavelength division multi/demultiplexing circuits connected together, wherein at least a second and the subsequent wavelength division multi/demultiplexing circuits are arrayed waveguide grating type wavelength division multi/demultiplexing circuits formed on a substrate and having a plurality of light input ends, a branching section, a branching section, a plurality of light output ends, and a plurality of arrayed waveguides sandwiched between the branching and branching sections so that incident light from each light input or output end is split and output from the respective light output or input end, and to an optical spectrum analyzer and an optical bandpass filter that use this wavelength division multi/demultiplexer.
2. Description of the Prior Art/Related Art
A wavelength division multi/demultiplexer for multiplexing and demultiplexing light is essential for a wavelength division multiplexing (WDM) system in optical communication. FIG. 1 shows an example of a configuration of a conventional wavelength division multi/demultiplexer. The wavelength division multi/demultiplexer is configured by one 16-input and 16-output arrayed waveguide grating type wavelength division multi/demultiplexing circuit (AWG) formed on a substrate 1 and having a plurality of light input ends 2, a first multiplexing and demultiplexing section (branching section) 3, a second multiplexing and demultiplexing section (multiplexing section) 5, a plurality of light output ends 6, and a plurality of arrayed waveguides 4 sandwiched between the branching section 3 and multiplexing section 5 to split incident light from each light input end 2 (#1, . . . , #16) and to emit it from an optical output end 6 (#1, . . . , #16). In this case, the light input and output ends refer to optical waveguides between the end of the substrate and the multiplexing and demultiplexing section. Of course, the reciprocity of light enables incident light from the light output ends 6 to be distributed to the light input ends 2. Light from a particular light input end is distributed to each arrayed waveguide 4 by the branching section 3. Each distributed light is again mixed together by the multiplexing section 5. The length of each arrayed waveguide 4 is configured by increasing at a constant pitch (referred to as .DELTA.L). In this figure, .DELTA.L=1,271 .mu.m and the number of arrayed waveguides 4 is 64. In FIG. 1, the 16 light input and output ends are named #1, #2, . . . , #16 from top to bottom in this order.
When a fixed light input and a fixed light output ends are used, only a series of optical frequencies offset from the light frequency of incident light by values called a free spectral range (simply referred to as "FSR") can be emitted from this light output end. FIG. 2 shows transmission spectra of two wavelengths that can be transmitted within a wavelength zone between 1549 and 1551.5 nm when the light input end #8 and the light output end #8 are used. This figure shows that the central wavelengths are 1549.7 and 1550.96 nm and that FSR=1.26 nm.
When the light output end is sequentially varied from #1 to #16 with the light input end fixed at #8, the transmission band sequentially shifts toward the long wavelength side as shown in FIG. 3. The amount of shift in the central frequency of the transmission band between adjacent light output ends is called a channel spacing, and is 0.08 nm, that is, 10 GHz in this AWG. The optical loss at the band peak is about 3 dB, and with a .+-.1 nm offset from the peak, the amount of transmission attenuates by 30 dB or more relative to the peak. As described above, due to its excellent band characteristics, the AWG is highly expected as a wavelength division multi/demultiplexer in wavelength division multi/demultiplexing communication. In the AWG, the multiplicity of the wavelength, that is, the number of channels is determined by the number of light output ends, and the number of channels in the illustrated AWG is 16. To increase the multiplicity, the number of light output ends must be increased. At present, however, the number of channels is limited to about 64. Thus, techniques for substantially increasing the number of channels in a wavelength division multi/demultiplexer using an AWG have been strongly desired.
The field of research and development of a WDM system in optical communication requires an optical spectrum analyzer having a spectrum resolution or an light frequency accuracy of about 1 GHz. The optical spectrum analyzer is an apparatus for measuring as a function of the light frequency the average optical power per unit light frequency over a certain time interval of output light from a light source. Most popular optical spectrum analyzers are diffraction grating optical spectrum analyzers using the spectral action of a diffraction grating. Due to the use of the dependence of the diffraction angle of the diffraction grating on the light frequency, this apparatus uses a motor to rotate the diffraction grating to vary a demultiplexing frequency in order to measure the power of diffracted light at each rotating angle. A movable portion of the diffraction grating is essential for such an optical spectrum analyzer and has degraded long-term stability, reduced the accuracy of measured optical frequencies, and increased the size of the apparatus. Besides, since the diffraction efficiency of the diffraction grating is low and only those of diffracted incident beams which have been transmitted through narrower slits are measured for power, (i) the entire apparatus is subjected to a large optical loss and (ii) the spectrum resolution obtained is at most about 0.08 nm, that is, 10 GH in a 1.5-micron band. Due to these backgrounds, there has been a strong desire for the development of a small high-resolution optical spectrum analyzer that has no movable portion, that is stable over a long time, and that has a high measured-frequency accuracy.
FIG. 4 shows an example of a conventional configuration of an optical spectrum analyzer using an AWG. In FIG. 4, 7 designates a light source, 8 designates an optical fiber, 9 denotes the 16-input and 16-output AWG shown in FIG. 1, and 10 denotes an array of 16 photodetectors attached to the light output end 6 of this division multi/demultiplexer. Optical output from the light source 7 is incident on the light input end #8 of the devision multi/demultiplexer 9 via the optical fiber 8. The power of light split by the multi/demultiplexer 9 is measured by the photodetectors 10. The wavelengths of demultiplexing are arranged at an equal space of 10 GHz as shown in FIG. 3, so the power per unit frequency can be measured for each light frequency using the central frequency of each band and the bandwidth .delta..nu.. Since the optical spectrum analyzer using the AWG spatially splits incident light so that the photodetector array can simultaneously measure the optical power from each light output end, it is characterized by its measuring time shorter than that of the conventional apparatus. In addition, since this analyzer has no movable portion as in the conventional apparatus and each bandwidth is small, that is, .delta..nu.=5 GHz, a small and stable optical spectrum analyzer of a high resolution is expected to be provided using the AWG.
Despite these excellent potentials of the optical spectrum analyzer using the AWG, there have been the following critical defects in the conventional configuration. A number of optical frequencies that can be emitted from a particular light output end of the AWG is not single, and there exists a periodicity with which a beam offset from the required beam by FSR=1.28 nm is also emitted from the same light output end, as shown in FIG. 2. When, for example, light from a light source having a bandwidth larger than the FSR is allowed to enter this AWG, as shown in FIG. 5, spectrum components of the light source having frequencies offset from the required frequency by an integer multiple of the FSR are transmitted from each light output end of the AWG as shown in FIG. 6. Thus, if the bandwidth of the light source is larger than the FSR, the spectrum of this light source cannot be measured by simply connecting the photodetectors 10 to the light output end 6 as shown in FIG. 4.
If this high-resolution optical spectrum analyzer is to be configured by using an AWG, the pitch between adjacent arrayed waveguides must be .DELTA.L&gt;2,000 .mu.m and the total number of arrayed waveguides must be 100 or more. Thus, the difference between the longest and shortest arrayed waveguides is 20 cm or more. It is very difficult to produce such a large-scale AWG on a conventional substrate.
The field of research and development of a WDM system requires a stable frequency-variable laser light source that has a variable light frequency. Such a light source is normally implemented by configuring a ring resonator comprising a semiconductor amplifier or fiber optic amplifier and an optical bandpass filter. Thus, optical bandpass filters have been strongly desired that have a stable transmission central frequency and a bandwidth or variable step in the order of GHz.
FIG. 7 shows a configuration of an optical bandpass filter the central wavelength of which can be varied by inclining a dielectric evaporation filter. Reference numerals 11 and 15 denote optical fibers, 12 and 14 are collimator lenses, and 13 is a dielectric evaporation filter. Output beams from the optical fiber 11 are made parallel with each other by the lens 12, propagate through the filter 13, and are incident again on the optical fiber 15 via the lens 14. When the filter 13 is rotated around an axis perpendicular to the sheet of this drawing, the angle at which output beams from the optical fiber 11 are incident on the filter 13 varies to vary the center of the transmission band of the filter 13, thereby enabling this configuration to achieve an optical bandpass with a variable transmission central frequency. A filter with a dielectric multilayer film, however, is disadvantageous in that its transmission bandwidth is at most 1 nm, that is, about 100 GHz in a 1.55 .mu.m band. Another disadvantage is that when the filter is rotated to increase the incident angle of light beams, the loss significantly varies in both directions in parallel with and perpendicular to the incident surface, thereby increasing PDL (that is, polarization dependent loss). Consequently, the implementation of variable optical bandpass filters having a bandwidth of about 10 GHz and a small PDL has been strongly desired.
FIG. 8 shows a configuration of an optical bandpass filter using the AWG 9 shown in FIG. 1. Reference numerals 16 and 18 indicate optical fibers and 17 is a 16.times.1 optical switch for propagating to the optical fiber 18 light distributed to one of the 16 light output ends #1 to #16 of the AWG 9. The optical fibers 16 and 18 are the light input and output ends of this filter. The optical switch 17 can be used to select one of the optical output ends and thus one of the 16 narrow band optical bandpass filters shown in FIG. 3. As in the optical spectrum analyzer using the AWG, however, the periodic characteristics of the AWG involves this filter with all optical bandpasses offset from the same light output end by an integer multiple of the FSR, so this filter does not provide a narrowband optical bandpass if the spectrum width of incident light is larger than the FSR.
As described above, a problem of the AWG is that it is very difficult for it to provide several hundred channels, and the development of new techniques for substantially increasing the number of channels in the AWG has been strongly desired. Another problem is that when the optical spectrum analyzer is to be configured by using the AWG, beams of wavelengths of frequencies offset from the required frequency by an integer multiple of the FSR pass through, so a large frequency range cannot be covered easily.
Furthermore, the AWG in which a channel spacing is narrow is indispensable in the optical spectrum analyzer using the AWG. However, this type inevitably becomes a large scale in accordance with the conventional configuration. Accordingly, there was such a large problem as to how this type could be downsized.
The conventional optical bandpass filter has a bandwidth of at most about 1 nm and a large PDL, so the development of optical bandpass filters that enable the band to be further narrowed and that provide a small PDL has been strongly desired. Although the AWG enables the transmission bandwidth to be reduced to 10 GHz or less, beams of wavelengths of frequencies offset from the required frequency by an integer multiple of the FSR pass through, so it is difficult to use the conventional configuration to provide an optical bandpass filter that can be operated over a wide frequency range.